Engineers use compression (also called coding or encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video by converting the video into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original video from the compressed form. A “codec” is an encoder/decoder system.
Most scalable video codecs split video into a base layer and an enhancement layer. The base layer alone provides a reconstruction of the video at a lower resolution, and the enhancement layer can be added to provide extra information that will increase the video quality. Some scalable codecs rely on temporal scalability of video. For example, certain frames of video are encoded by the encoder, but the decoder does not reconstruct those frames. Other common scalable encoding schemes involve scalability for either the spatial resolution or the signal-to-noise ratio (“SNR”) of the video.
In many prior art systems, an enhancement layer codec depends on the base layer to provide a certain amount of information necessary to encode or decode the enhancement layer. For example, the enhancement layer coding is based upon base layer coding information such as motion vectors and DCT coefficients used to encode the base layer frame. As a result, the enhancement layer codec is coupled with the given base layer codec being used so the enhancement layer codec can use the same macroblocks and motion vectors as the base layer codec. Thus, a base layer encoded according to one standard would require a different enhancement layer codec than a base layer encoded according to another standard. This multiplicity of enhancement layer codecs can lead to a very high cost of developing, testing, and deploying the enhancement layer codecs.
Scalable video codecs that support temporal scalability, spatial scalability and/or SNR scalability provide many options for base and enhancement layers. While these types of scalability provide acceptable performance in many scenarios, they do not have the benefits and advantages of the techniques and tools described below.